The prior art electronic control system for an automatic transmission provides various electronic controls, such as upshifts and downshifts employing solenoid valves, engagement and disengagement of the clutch of the lockup torque converter, control of the line pressure, and control of the engine torque.
In the prior art automatic transmission, the line pressure for engaging the clutch that produces a shift or for applying the brakes is established, using the throttle opening as a parameter, whether the transmission is controlled electronically or hydraulically. The throttle opening is used, because it represents the characteristics of the engine torque. Therefore, the hydraulically controlled automatic transmission uses a throttle cable. On the other hand, in the electronically controlled automatic transmission, the throttle opening is transformed into an electrical signal, and the line pressure is adequately controlled. In this way, a suitable feeling is given to the driver of the automobile when a shift is produced.
In recent years, vehicles have begun to be equipped with a traction control system which permits the vehicle to go while reducing the torque by a given amount, in order to improve the operating performance and the safety of the vehicle on bad roads and under bad weather conditions. This system is disclosed in Japanese Patent Laid-Open No. 117443/1990. When a vehicle equipped with this traction control system is running on a gravel road, if the driving wheels skid and the driver keeps pushing down on the accelerator pedal, the engine torque is automatically reduced. This assures that the vehicle runs smoothly.
When a vehicle equipped with the above-described electronically controlled automatic transmission is going on a road surface of a small friction coefficient such as a snowy road, if the driving wheels skid, the engine torque is reduced by the traction control function. At this time, the automatic transmission may not make a smooth shift, producing a shock.
Specifically, the signal produced from the throttle sensor used in the prior art system for the automatic transmission indicates the position of the accelerator pedal that the driver presses down on. The throttle opening remains the same if the position of the accelerator pedal is not shifted, whether the traction control function is being performed or not.
When the driving wheels skid, the output torque from the engine is decreased actually by the control of the traction. In spite of this decrease, the accelerator pedal remains depressed, thus producing the same line pressure. The automatic transmission is controlled by this constant line pressure.
Essentially, the line pressure should be set to appropriate values according to the output torque from the engine. As described above, when the traction control function is performed, excessive line pressure is applied to a hydraulic servo such as the clutch for producing shifts or the brakes and so a shift is made in an abnormally short time, thus producing a great shock.
When the wheels are skidding, if the driver further presses down on the accelerator pedal, a downshift is produced. Then, the driving force is increased, leading to an increase in the amount of skid. This makes it impossible to appropriately control the traction.